This invention relates to a method for isolating difluoromethane (xe2x80x9cHFC-32xe2x80x9d or xe2x80x9c32xe2x80x9d) from a crude mixture containing hydrogen chloride (xe2x80x9cHClxe2x80x9d) and an azeotropic mixture of 32 and hydrogen fluoride (xe2x80x9cHFxe2x80x9d), particularly, as part of a process wherein 32 is being manufactured by reacting methylene chloride (xe2x80x9c30xe2x80x9d) and HF, such reaction normally being conducted in the gas phase. The reactor effluent from such reactions mostly comprises 32, unreacted 30 and HF, by-product HCl and an intermediate product chlorofluoromethane (xe2x80x9c31xe2x80x9d). HFC-32 is a known refrigerant and is typically used blended together with other refrigerants, such as pentafluoroethane (xe2x80x9cHFC-125xe2x80x9d).
A number of schemes have been proposed for purifying the 32 contained in this reactor effluent, some of which cite a low HF concentration azeotrope with 32 as precluding isolation of 32 by straight distillation. In U.S. Pat. No. 5,707,497, the azeotrope is broken by pressure swing distillation. In U.S. Pat. No. 5,523,015, the azeotrope is broken by phase separation. Both of these methods require a substantial investment and isolation of HF azeotropes. It would be useful to have a scheme for isolation of 32 which does not require isolation of an azeotrope.
A method of isolating 32 from a crude mixture containing HCl and an azeotropic mixture of 32 and HF is provided, which method comprises the steps of (A) distilling said crude mixture in a column with a cofeed of a compound (preferably 30) having a boiling point higher than that of 32 in order to generate a column overhead of HCl and a column bottoms whose bubble point and composition varies with the amount of heat applied to it, (B) removing HF from the column bottoms of step (A) by washing, and (C) distilling the remaining mixture from step (B) to separate 32 from higher boiling compounds. The higher boiling compounds from step (C) are preferably recycled to the column of step (A). As described in more detail below, the xe2x80x9cwashingxe2x80x9d referred to in step (B) encompasses both absorbing into water and neutralizing with aqueous caustic.